As is well known, at present, the bodies of many automobiles are monocoque bodies in which a load is supported by the overall body which is integral with a frame in order to achieve both a decrease in weight and high stiffness. The body of an automobile must be able to suppress impairment of the functions of the vehicle at the time of a collision of the vehicle and protect the lives of passengers within a passenger cabin. In order to decrease damage to a passenger cabin by absorbing the energy of impact at the time of a collision of a vehicle and reduce the impact force to the passenger cabin, it is advantageous to preferentially crash spaces other than the passenger cabin, such as the engine compartment or the trunk.
On account of such safety demands, crash energy absorption members which actively absorb impact energy by collapsing when an impact load is applied at the time of a collision are provided in suitable locations, such as at the front, the rear, or the side of a vehicle. Examples of such crash energy absorption members are cross side members, side sills, and rear side members.
In recent years, it has been attempted to increase the safety of vehicles and to reduce repair costs by nearly eliminating damage to vehicles caused by light impacts by mounting a crash energy absorption member referred to as a crash box on the front end of a front side member by a suitable means such as joining with a connector or welding. A crash box is a member which absorbs impact energy by preferentially buckling in the axial direction into the shape of a bellows (accordion) under an impact load which is applied in the axial direction (in this specification, this means the lengthwise direction of a crash energy absorption member).
Various materials and shapes have thus far been developed for increasing the crash energy absorbing performance of such a crash energy absorption member. The crash energy absorbing performance which is demanded of a crash energy absorption member are, specifically, that it deform into a bellows shape by repeatedly stably buckling in the axial direction when an impact load is applied in the axial direction, that the average load be high at the time of collapse of the crash energy absorption member, and that the largest reaction force which is generated upon the collapse of the crash energy absorption member be within a range which does not cause damage to other members disposed in the vicinity of the crash energy absorption member.
Up to now, crash energy absorption members which have generally been used have been box-shaped members welded to a backing plate by means of a flange provided on a member having a hat-shaped transverse cross-sectional shape like that disclosed in JP-A 08-128487, for example. In this specification, “flange” means an edge portion which projects outwards from an outline of a transverse cross section.
In contrast, in JP-A 09-277953, an invention pertaining to a crash energy absorption member is disclosed which decreases the load at the initial stage of a collision and increases the absorbed impact by having a closed cross-sectional structure such that the transverse cross-sectional shape continuously changes from one end towards the other end from a polygon having at least 4 sides to a polygon having a larger number of sides.
JP-A 2003-48569 discloses an invention pertaining to a crash energy absorption member which has a polygonal closed cross-sectional shape with a partition in its interior.
JP-A 2002-284033 discloses an invention pertaining to a crash energy absorption member which secures strength by forming a groove with a generally right triangle shape facing towards the interior in a region including each of 4 vertices of a material having a rectangular transverse cross section.
In addition, JP-A 08-108863 discloses an invention which forms a bead which extends in the axial direction on the side surface of a front side frame having a hat-shaped cross section with a flange in order to suppress bending of the front side frame when an impact load is applied.
However, in any of these existing inventions, a crash energy absorption member cannot be provided which can secure a prescribed amount of shock absorption by stable buckling in the axial direction without leading to an increase in weight due to the addition of a partition or an increase in plate thickness.
The transverse cross-sectional shape of crash energy absorption members used in the bodies of automobiles is in almost all cases flat. Therefore, it is difficult to use a crash energy absorption member having a polygonal transverse cross-sectional shape such as a simple regular polygon as disclosed in JP-A 09-277953. In the invention disclosed in JP-A 09-277953, the transverse cross-sectional shape of the crash energy absorption member gradually changes over approximately its entire length. Therefore, in some positions in the axial direction, the transverse cross-sectional shape of the crash energy absorption member may unavoidably become a shape which is not suitable for stable buckling. Accordingly, when an impact load is applied in the axial direction, that crash energy absorption member cannot stably repeatedly buckle in the axial direction, and it may not deform into a bellows shape.
In the invention disclosed in JP-A 2003-48569, there is the possibility of the strength of the portion in which a partition is provided increasing too much. Therefore, in that invention, buckling may become unstable and the amount of shock absorption may become insufficient. There is also the possibility of the largest reaction force which is generated in the crash energy absorption member, particularly in the initial stage of collapse, exceeding the strength of other members and of the other members collapsing before the crash energy absorption member collapses. In addition, in that invention, the weight of the crash energy absorption member unavoidably increases by an amount corresponding to the partition provided in its interior. Therefore, that invention is contrary to the trend towards decreasing the weight of vehicle bodies, for which there has been a particularly strong desire in recent years.
In the invention disclosed in JP-A 2002-284033, corner portions which inherently have a high strength undergo further working to form a notch therein. As a result, the strength of the notch may increase too much, and it may become impossible for stable buckling to take place. Accordingly, in that invention, in the same manner as in the invention disclosed in JP-A 2003-48569, the amount of shock absorption may be insufficient, and other members may collapse before the crash energy absorption member collapses.
In the invention disclosed in JP-A 08-108863, a crash energy absorption member has a hat-shaped transverse cross-sectional shape with a flange. According to that invention, it is thought that it is indeed possible to suppress bending due to an impact load applied thereto. However, according to that invention, when an impact load is applied, it cannot stably collapse in the axial direction into a bellows shape.
An object of the present invention is to provide a crash energy absorption member which can secure a prescribed amount of shock absorption by stably buckling in the axial direction into the shape of a bellows when an impact load is applied thereto without an increase in weight due to the addition of a partition or an increase in plate thickness and without causing bending in the axial direction.